1. Field of the Invention
The present invention relates to a liquid ejection head which generates and provides energy to eject a liquid through ejection ports in the liquid ejection head, and to a liquid ejection method for ejecting a liquid from the liquid ejection head.
2. Description of the Related Arts
Presently, a method using a heat generating element to eject ink is widely utilized for inkjet printing apparatuses. According to this method, ink is supplied along flow paths to a common liquid chamber, and when this chamber is filled, an electric signal is applied to a heat generating element to generate heat. The heat generating element is arranged in a bubble generation chamber to serve as an energy application chamber, thereby initiating the production of heat. Thereafter, ink around the heat generating element in the bubble generation chamber is heated rapidly to the boiling point, i.e., is boiled, and forms a bubble on the heat generating element. As a consequence of this phase change, an increased pressure generated as a result of production of the bubble, imparts to the ink in the bubble generation chamber sufficient kinetic energy to eject ink outward and to eject an ink droplet to the exterior, through an ejection port. Thus, thermal energy applied to the ink by the heat generating element is converted into kinetic energy, which in turn causes an ink droplet to be ejected. As a consequence, as ink droplets are ejected through ejection ports which are in communication with bubble generation chambers, the printing is performed to a printing medium. Furthermore, since this type of printing apparatus is simply structured, one of its more notable features is that the ink ejection arrangement provides easy means for the integration of ink flow paths, for example.
When this ink ejection method is utilized, a bubble generated by a heat generating element grows until ink is ejected. Thereafter, heat retained by the heat generating element and ink in the vicinity of the heat generating element is dispersed to reduce the volume of the bubble. Then, for disappearance of the bubble, collapse of the bubble is caused by ink in the bubble generation chamber. This collapse of the bubble may cause surface damage within the bubble generation chamber. That is, surface cavitation may occur, and consequently, with the driving of the heat generating element, may damage the surface of the heat generating element. Therefore, as a countermeasure, to maintain durability and to ensure availability for practical use is not impaired, a protective layer, such as one composed of Ta, is deposited on the surface of the heat generating element.
As another countermeasure for avoiding cavitation damage, proposed, for example, is a print head disclosed in Japanese Patent Laid-Open No. 2002-321369. According to this proposal, a print head is disposed wherein the center line of a heat generating element is offset relative to the center line of an ink flow path leading to a bubble generation chamber. Since, in this manner, the center line of the heat generating element is shifted away from the center line of the ink flow path. Thus, it is prevented that a location at which bubbles are disappeared is concentrated at a single location. Therefore, the locations at which cavitation may occur can be scattered. This also prevents disappearing bubbles at locations around the heat generating element. As a result, since the location at which a bubble may disappear will not correspond to a heat generating element, cavitation occurring at locations around the heat generating element surface is prevented, and damage to the heat generating element is avoided.
Furthermore, according to an ink ejection method disclosed in U.S. Pat. No. 6,155,673, when a bubble has grown and ink ejection is imminent, the bubble is permitted to communicate with external air. According to this ink ejection method, since a path from the bubble to the exterior is opened, the internal bubble pressure is vented externally, abruptly dropping until nearly equivalent to that of the air. Thus, the bubble is released to the air without collapsing by ink, and ink is supplied, in an amount of ink equivalent to that ejected, to refill the bubble generation chamber. Therefore, since it is inhibited that the bubble remains in the bubble generation chamber in this manner, cavitation occurring is inhibited, and damage to the surface of the heat generating element can be prevented.
Moreover, another ink ejection method whereby a bubble is permitted to communicate with external air, as in U.S. Pat. No. 6,155,673, is proposed in U.S. Pat. No. 6,354,698. According to this method, first, a bubble is permitted to grow until a maximum bubble volume is reached while ink is being ejected, and then, at the succeeding step of the bubble volume is reduced, it is permitted the bubble to communicate with external air. When this method is used to perform ink ejection, not only cavitation occurring is inhibited, as with the preceding method, but also, after ink has been ejected, the liquid surface at the ejection port recedes in a direction opposite that in which ink is ejected. Thus, ink that may form a satellite droplet is easily separated from the main ejected droplet, and absorbed by the surface of liquid at the ejection port. As a result, the occurrence of mist is prevented, and high quality printing enabled.
When the liquid ejection method of an air communication type, as proposed in U.S. Pat. No. 6,155,673 or U.S. Pat. No. 6,354,698, is used, occurrence of cavitation is inhibited. The occurrence of cavitation, however, is not fully prevented by using these liquid ejection methods, and depending on the case, cavitation may still appear.
While referring to FIGS. 12A to 12F, an explanation will now be given for an example ink ejection process performed by an ink ejection method, as proposed in U.S. Pat. No. 6,354,698, whereby at first, a bubble grows, attaining a maximum bubble volume while ink is being ejected, and then, at the succeeding step for reduction of the bubble volume, the bubble is permitted to communicate with external air.
As shown in FIG. 12A, when based on a print signal, for example, a current is supplied to a heat generating element and a bubble is thereby generated in an ink flow path, then the bubble abruptly inflates and grows rapidly. Then, as shown in FIG. 12B, in response to a pressure buildup, the result of the bubble generation, ink is ejected through an ejection port. While the ink ejection process is carried out, simultaneously, a maximum bubble volume is reached, and thereafter, as shown in FIG. 12C, the volume of the bubble is reduced. At nearly the same time, inside the ejection port, formation of a meniscus is begun. Since the amount of ink in a bubble generation chamber is reduced when ink is ejected, as shown in FIG. 12D, the meniscus moves inward, toward the heat generating element. Since the meniscus travels at a higher speed than that at which bubble deflation occurs, as shown in FIGS. 12E and 12F, the meniscus catches up with the still inflated bubble, which can then communicate with air below the ejection port. At this time, communication between the bubble and the air occurs at a location near the center of the heat generating element.
In a case such as shown in FIG. 12D, where the meniscus is moving toward the heat generating element, the surface of liquid traveling toward the heat generating element pushes against and compresses both the ink situated between the meniscus and the heat generating element and the bubble portion. Therefore, while being compressed, substantially toward the center of the heat generating element, the bubble is bent and the portion opposite the center of the heat generating element is formed into an annular shape. Sequentially, thereafter, as shown in FIG. 12E, the bubble having the annular portion is divided into a portion nearer the rear wall of the heat generation chamber and a portion nearer the ink supply port. Since the divided bubble of the portion nearer the ink supply port which has the larger volume is in communication with air, the internal bubble pressure is reduced to that of the atmosphere. Then, new ink is supplied to the bubble generation chamber, the bubble generation chamber is refilled, the bubble portion is in communication with the air, and the bubble disappears, as shown in FIG. 12F, while the communication state is maintained. However, since no bubble to air communication is established for the bubble portion near the rear wall of the bubble generation chamber, that bubble portion remains in the bubble generation chamber and may cause cavitation. As described above, it was found that when bubble to air communication is established near the center of the heat generating element, the bubble tends to be divided, and since a bubble portion for which bubble to air communication is not established is not removed, cavitation may occur. Further, since cavitation may occur, the protective layer formed on the surface of the heat generating element would be damaged, and the durability of the heat generating element deteriorated.
In addition, a behavior of phenomenon is changed depending on the height of an ink flow path formed in a bubble generation chamber, the phenomenon is that once a maximum bubble volume is reached, and then, when the volume of the bubble is reduced, bubble to air communication is established. The greater the height of an ink flow path in a heat generation chamber, the smaller the difference is obtained between the respective speeds at which a meniscus travels after ink is ejected and at which a bubble deflates. Therefore, the period required to establish bubble to air communication is extended. Thus, the successful accomplishment of this event is delayed. The establishing bubble to air communication is carried out with the compression and deflation state of the bubble, in this case, is more advanced. As a result, bubble division tends to occur more frequently, and the possibility is greater that a bubble portion will remain in a bubble generation chamber and cause cavitation.